The present invention relates to a negative electrode used for a nonaqueous electrolyte solution battery having excellent charge-and-discharge cycle characteristic, and a nonaqueous electrolyte solution battery having the negative electrode.
In recent years, a cordless electrical equipment such as a video camera or portable telephone has a remarkable development. As a power source of the cordless electrical equipment, a lithium secondary battery having a high battery voltage and a high energy density attracts attention. As a material for activating a negative electrode of the lithium secondary battery, a carbon material able to store or emit lithium such as hard-to-graphitized carbon or graphite is used, because of a comparatively high capacity and good cycle characteristic.
However, the active material is required to have a further higher capacity, because the electrical equipment is further required to be small and to be used for a long time.
JP-A-07-29602 discloses a negative electrode active material such as lithium metal or lithium alloy so as to make a lithium secondary battery to have a higher capacity. Moreover, in recent years, simple metal such as Si, Sn or Al, or its alloy is proposed as the negative electrode active material.
However, a metal material such as Si or Sn has a large volume change in an alloying reaction with lithium, at a time of charging and discharging. In this case, the active material may have a crack or sliding. If an electric conduction route of the active material is lost by the crack or sliding, a cycle life for charging and discharging becomes short.
JP-A-2006-260886 discloses a solution for this disadvantage. In JP-A-2006-260886, alloy or metal to be alloyed with lithium has porous structure. The porous structure is produced by depositing polymer particles such as polystyrene or PMMA, by plating metal to be alloyed with lithium to the deposited particles, and by eliminating the particles. By this method, volume expansion generated in a charging time can be absorbed since many uniform communication apertures are formed. However, electrode itself becomes weak by the porous structure, and the weakness is shown in an embodiment of JP-A-2006-260886, in which the electrode is evaluated in a glass cell without forming a battery.
JP-A-2004-220910 discloses a mixture of carbon nano-fiber having aspect ratio of 10 or more and a metal-based active material to be alloyed with lithium. In this case, particles of the active material can be connected with each other. Therefore, a sliding of the active material and an exfoliation of electrode can be prevented, even if a volume change is generated in a charging and discharging time. In this case, since the carbon nano-fiber is twisted around the metal-based active material, electric conduction route can be secured. However, the negative electrode active material has a substantial volume change in a charging and discharging time, as shown in a cross-sectional view of JP-A-2004-220910 illustrating a volume expansion of an inorganic particle (negative electrode active material) when the inorganic particle is added into a carbon material. Therefore, the crack or the sliding of the active material is similarly generated even if a carbon material having fibrous or particle state is located around the metal-based active material having several-μm level. Although a remarkable difference (effect) can be seen if a period is short such as ten cycles, it is difficult to withstand charging and discharging cycles over a long period.
Further, JP-A-2007-335283 discloses a conducting material having a diameter smaller than an average pore diameter of a porous conductive material, made of the metal-based active material to store or emit lithium. A metal mesh having several or tens μm-mesh, or a nonwoven fabric or felt made of carbon material is used as a mold of the porous material. That is, a material disclosed in JP-A-2004-220910 is also suggested by JP-A-2007-335283. In this case, an effect of holding the whole electrode can be expected other than the effect shown in JP-A-2006-260886. However, similarly to JP-A-2004-220910, the crack or the sliding of the active material is similarly generated even if a carbon material having fibrous or particle state is located around the metal-based active material having several-μm level.
JP-A-2009-272041 discloses a lithium-ion secondary battery, in which minute carbon fibers adhere to particle surfaces of positive electrode active material and negative electrode active material in a mesh state. JP-A-2003-100284 discloses a negative electrode, in which inorganic compound (however, carbon is excluded) adheres to outer and inner surface of a pore of a porous carbon so as to store or emit lithium ion.
Moreover, performances of the above inventions are required to be further improved.